The exemplary embodiment relates generally to the gloss inherent in the hardcopy of image data, be it pictorial or text. More particularly, it relates to halftoned image data and the control of differential gloss when that halftone image data is printed into hardcopy.
Digital watermarks have been applied to images for a variety of purposes, including verification of authenticity of documents, providing information, and prevention of copying. Many of the techniques for generating such watermarks involve adding image data or modifying image data in an existing image prior to printing the image. In other techniques, selective application of a clear toner to a rendered image is used to create a watermark.
More recently, Glossmark™ technology based on differential gloss has been used for the generation of digital watermarks. The technology allows a digital watermark to be introduced without modification of the original image content or use of special marking materials or substrates. Rather, the technique creates a differential gloss, image which can be discerned by a human reader holding the printed paper at an angle but which cannot be detected by a scanner or copier during normal scanning. These devices are restricted to reading the image at a fixed angle to the paper, which is generally selected so as to minimize the impact of specular reflection. The differential gloss image is created by selective use of two different halftone screens which create two different gloss characteristics in halftone outputs.
In halftone printing, a contone image (an image in which gray levels are-substantially continuously variable) is converted to a binary image before printing. At a specified location, or pixel, a spot is either printed or not printed, which is controlled by halftone techniques. The most common halftoning method is screening, in which a halftone screen determines how a cluster of pixels grows with increasing gray level. Here, gray level refers to increasing intensity for any channel, irrespective of whether it is a monochrome or a color channel. The halftone screen compares the required continuous tone levels with predetermined threshold levels typically defined for a rectangular cell that is tiled to fill the image plane. The output of the screening process is a binary pattern of multiple small “dots,” which are regularly spaced in a grid as is determined by the size, shape, and tiling of the halftone screen. In conventional screening process, the dots grow generally radially outward as gray level increases to form circular-shaped clusters. In current Glossmark™ technology, two halftone screens having different anisotropic structure orientations are used. The anisotropic structure may be provided by the growth pattern and/or orientation of the screen. By toggling between the two halftone screens in generating the halftone image, differences in gloss are apparent when the rendered image is viewed at an angle.
Although there are many different screen combinations which provide desired gloss difference for the differential gloss effect, matching the color or density appearances of two different halftone structures usually requires two separate color calibrations for two screens. Without color calibration, the halftone outputs corresponding to the two halftone screens may well be distinguishable not only in their gloss difference but also in their color and/or density. Even with such color calibrations, machine defects, such as banding, miss-registration, or distortion, may cause color miss-match for two different halftones from time to time or from place to place. To minimize these effects, two halftone structures which are mirror images of each other may be selected. This generally avoids the need for two separate color calibrations, but also provides a robust solution for creating Glossmark™ images which will be apparent only in their gloss difference, and not in color and/or density. As a consequence, differential gloss images which provide consistent color and density have, to date, been formed with only two distinguishable gloss levels.